Description ⁴

Large, finger-sized slugs ranging from bright banana yellow to dull green with mottling. Given their size and color they are pretty easy to distinguish from our other local slug groups. Identifying beyond the genus, though, is tricky...

Taxonomy ⁴

Banana slug taxonomy seems to be in a state of taxonomic limbo, at present. There has been abundant work done on banana slug genomics at UC Santa Cruz, much of which is present (in varying degrees of organization) at https://banana-slug.soe.ucsc.edu. Leonard et al. (2007) summarize the historical state of this group in our area (see "Taxonomy" under "Materials and Methods"), but basically, prior to molecular techniques, San Franciscan banana slugs would have been called Ariolimax columbianus or Ariolimax californicus ssp. brachyphallus, based primarily on penis morphology. However, modern molecular data from Pearse and Leonard suggests that the old Ariolimax columbianus concept disguises several genetically distinct species. Sadly, I cannot find a publication in a peer-reviewed journal that presents this data and definitively establishes these new species concepts in a manner concordant with the rules of the International Commission on Zoological Nomenclature, and the best (really only) source for these findings is "All About Banana Slugs", a PDF apparently from 2010 that seems to be a deck of slides presented to a university class. Thus, while it seems from the outside that much of this work has been done, the modern treatments have not technically been officially accepted according to ICZN rules.

So, if you want to stick to the published literature, then there are at least two species of banana slugs in San Francisco, A. columbianus and A. californicus ssp. brachyphallus, the former generally being larger and darker and the latter being smaller and more lemon yellow, but only reliably distinguished based on genital morphology.

But, if you think adherence to ICZN and peer-review is too high a bar and you trust the "unpublished" work of Pearse and Leonard we can glean from the web (most significantly in "All About Banana Slugs" (2010)), then we would call these slugs A. buttoni and A. brachyphallus, with some potential for A. brachyphallus x californicus hybrids, but again, the two groups would only reliably be separated by genital morphology and/or genetic analysis.

I think the main upshot for field identification is that it is basically impossible when there are multiple species reported in an area, and San Francisco is such an area. It seems probable that the larger slugs with spots are A. columbianus / buttoni and those without could be either species, but I haven't seen any statements in the literature that would confirm that.

In San Francisco ⁴

Whatever you want to call these slugs, it's kind of astounding how well they've survived human occupation of their land! Banana slugs can still be found in decent numbers in the Presidio, and they've been sighted in many other wild to semi-wild areas in the city, including Mt. Sutro, Twin Peaks, Glen Canyon, and Lake Merced. You're best chance of seeing them will be to go for a walk in forested areas in any of these places after a good rain shower. They'll happily crawl around during daylight hours, and some are very bright yellow, so they stick out!

Sources and Credits

1. (c) Such A Groke, some rights reserved (CC BY-NC-SA), http://www.flickr.com/photos/70832699@N00/617253640
2. (c) brewbooks, some rights reserved (CC BY-SA), https://www.flickr.com/photos/brewbooks/2732040662/
3. (c) Lynette, some rights reserved (CC BY-NC-SA), https://www.flickr.com/photos/25980517@N03/2980246732/
4. (c) Ken-ichi Ueda, some rights reserved (CC BY-SA)

More Info

• iNat taxon page
• Biodiversity Heritage Library
• BOLD Systems BIN search
• Global Biodiversity Information Facility (GBIF)
• World Register of Marine Species

iNat Map